The extracellular matrix of animal connective tissues Flashcards
The classes of macromolecules that constitute the ECM are broadly similar, but it is the variation in their ___________and their _________ that give rise to diversity of tissues.
Relative amounts, organization
The ECM can become
can become calcified to become bone or teeth or it can become the transparent substance of the cornea or the rope-like organization of tendons
Matrix molecules are secretes by cells called
fibroblasts
In certain specialized connective tissues such as cartilage and bone, fibroblasts have specific names such as ________ and ________, respectively.
Chondrocytes, osteoblasts
The matrix in connective tissue is constructed from the same two main classes of macromolecules as in basal laminae:
1) glycosaminoglycan (GAG) polysaccharide chains usually covalently linked to proteins in the form of proteoglycans
2) fibrous proteins such as collagen.
Polysaccharides
resists compressive forces on the matrix while permitting rapid diffusion of nutrients, metabolites, and hormones between the blood and tissue cells
Collagen fibers
strengthen and help organize the matrix, while other fibrous proteins such as elastin (rubber-like) give it resilience.
Major protein of the extracellular matrix
Collagens
Collagens are secreted in large quantities by _______ and small quantities by _______
Connective tissue cells, many other cell types
What is the principal collagen of skin and bone
Type I collagen
After being secreted into the extracellular space, these collagen molecules assemble into higher order polymers called
collagen fibrils
Primary feature of a typical collagen molecule
its long, stiff, triple-stranded helical structure, in which three collagen polypeptide chains called α chains are wound around one another in a rope-like superhelix.
What are collagens extremely rich in that are found every 3rd a.a
Glycine
What does glycine allow for
allows the three helical α chains to pack tightly together to form the final collagen superhelix
What is found under the fibroblast cell
Long strands of collagen fibrils
Post-translational modifications collagen chains undergo
Individual collagen polypeptide chains are synthesized on membrane-bound ribosomes and injected into the lumen of the rough endoplasmic reticulum as large precursors called pro-α chains. After being trimmed, hydroxylated and glycosylated, each pro-α chain then combines with two others to form the triple-stranded, helical procollagen
________ and ________ are infrequently found in other animal proteins
Hydroxylysines and hydroxyprolines
What gives tissues their elasticity
Elastin
Vertebrate tissues need to be both ______ and ______
strong, elastic
What makes vertebrate tissue strong and elastic
Strong: collagen
Elastic: elastin
A network of elastic fibers in the ECM of these tissues
gives them the required __________- so that they can recoil after transient stretch.
Resilience
How is stretching and tearing limited
Long, non-elastic collagen fibrils are interwoven with elastic fibers
Like collagen, elastin is
unusually rich in proline and glycine
Unlike collagen, elastin is
not glycosylated and contains hydroxyproline but not hydroxylysine
The precursor of elastin
Soluble tropoelastin
How are elastic fibers assembled at the PM?
Soluble tropoelastin is secreted into the extracellular space and assembled into elastic fibers
What happens after secretion of tropoelastin
Tropoelastin becomes highly crosslinked to one another between lysines
The elastin protein is composed largely of WHAT two types of short segments that alternate along the polypeptide chain
1). hydrophobic segments which are responsible for the elastic properties
2 alanine- and lysine-rich alpha-helical segments which form cross-links between adjacent molecules
Fibronectin
an extracellular protein that helps cells attach to the matrix
The ECM contains a number of
non-collagen proteins with
binding sites for other matrix macromolecules and for cell-surface receptors
Fibronectin is found where an is important for
large glycoprotein found in all vertebrates and important for many cell-matrix interactions
What happens to mice that cant synthesize fibronectin
die early in embryogenesis because their endothelial cells are unable to form proper blood vessels.
Fibronectin is a dimer composed of
two very large subunits joined by disulfide bonds at one end
What regulates the assembly of fibronectin fibrils
Tension exerted by cells
Fibronectin in soluble form
Circulates in the blood and other bodily fluids
Fibronectin in insoluble form
Fibronectin dimers are cross-linked to one another by additional disulfide bonds in the ECM
Difference between fibrillar collagen and fibronectin molecules (form and assembly)
Fibrillar collagen can form fibrils in test tubes alone, fibronectin molecules only assemble into fibrils on the surface of cells and only where the cells possess appropriate binding partners such as integrins
Integrins provide
A linkage from the fibronectin outside of a cell to the actin cytoskeleton inside
Fibronectin binds to cells through an
RGD motif that bind integrins
What molecules assemble into fibrils where cells possess integrins or binding partners
Fibronectin
The ability to cut through the matrix is crucial in two ways
1) it enables cells to divide while embedded in the
matrix;
2) it enables them to travel through it
Matrix degradation is exploited by __________ to spread through the body
Cancer cells
Cells degrade matrix components through extracellular ___________ that act close to the cells that produce them
cancer cells
Many of these proteases belong to one of two general classes:
- 1) matrix metalloproteases, which depend on bound
Ca 2+ and Zn2+ - 2) serine protease, which have a highly reactive serine
in their active site
Some are highly specific and only cleave particular
proteins in a few sites such as _________
collagenases
Less specific proteases
Act just where needed as they are anchored to the plasma membrane
We can restrict the breakdown of the ECM by
- Having the protease be very specific in which type of ECM it is going to break down
- Limit its location by anchoring the protease to a specific structure
Local activation
Many proteases are secreted as inactive
precursors that can be activated when needed.
Plasminogens are inactive proteases in the blood that,
when cleaved by plasminogen activators, yield the active protease plasmin which helps break up blood clots.
Confinement by cell-sruface receptors
Many cell-surface
receptors bind to proteases confining the enzyme to
where it is needed. An example is urokinase plasminogen
activator which is found at the leading edge of migrating cells. (limited by location)
Secretion of inhibitors
Secreting protease inhibitors such as tissue inhibitors of metalloproteases (TIMPs) and the serine protease inhibitors known as serpins.
The matrix exerts powerful influences on the cells through
Transmembrane cell adhesion proteins that act as matrix receptors
The principal receptors on animals cells for binding
most ECM proteins are the
Integrins
Integrins bind
Laminins and fibronectins
Integrins (large family of transmembrane adhesion molecules) have the ability to
transmit signals in both directions across the cell membrane
Integrins are _______ that link to the cytoskeleton
Transmembrane heterodimers
An integrin molecule is composed of
Two non-cavalently associated glycoproteins called alpha and beta
Integrin: The __________ portion binds to ECM proteins such as laminin or fibronectin, or to ligands on the surface of other cells
Extracellular
Integrins: the intracellular portion binds to _______ via
Binds to actin via the protein talin and a set of anchorage proteins
True or false: integrins can switch between an active and an inactive conformation
True
What does a cell crawling through a tissue have to be able to do
has to be able to make and break attachments to the matrix and to do so rapidly if traveling quickly.
If force is to be applied where it is needed, the
making and breaking of attachments has to be
coupled to the assembly and disassembly of
cytoskeletal attachments inside the cell.
As an integrin binds to or detaches from its ligands,
it undergoes _________________ that affect
both the intracellular and the extracellular end of
the molecule.
conformational changes
Outside-in integrin activation
Stimulus binds to the RGD domain of fibronectin, inducing a structural change that causes the talin binding domain to be revealed which allows the integrin (and ECM via fibronectin) to be linked to the cytoskeleton inside the cell
Talin links
The integrin to the cytoskeleton
Inside-out integrin activation
In _________ and ______, the ability to regulate integrin activity via inside-out signalling is particularly important
White blood cells, platelets
Repeated adhesion allows
A cell to circulate
unimpeded until it encounters an appropriate
stimulus
True or false: Integrins do not need to be synthesized de novo therefore the signal is rapid
True
True or false: In inside-out integrin signalling, the signal could still have originated from outside the cell BUT the activation of the integrin specifically is going in an inside out order
True
Inside-out signalling PIP2
PIP2 activated Talin, Talin binds b-integrin, conformational change to open integrin, binds ECM
Integrins differ from RTK’s in that
they bind their ligand with lower affinity and are therefore more abundant (10-100-fold) at the cell surface
The Velcro principle
Many receptor ligand interactions give increased
strength to the adhesion between cell and ECM
–> multiple low affinity bonds cause higher strength
ECM attachments act through ________ to control
cell proliferation and survival
Integrins
Integrins activate ______________ to allow a cell to behave according to its
surrounding matrix
Intracellular signalling pathways
What happens when cells lose contact with their ECM
They initiates apoptosis
ANCHORAGE-DEPENDENCE
FAK is recruited by
intracellular anchor proteins such as talin
How is docking site for the Src family of tyrosine kinases (Fyn)
The clustered FAK molecules cross-phosphorylate each other on a specific tyrosine
What does FAK do
FAK helps disassemble focal adhesions
Outside-in signaling is relayed from integrins, via
_____ and ________- kinases into the cell.
FAK and Src-family kinases
Mutant mice lacking FAK
their fibroblasts still adhere to fibronectin and form focal adhesions. But they
form too many focal adhesions and do not migrate
